The invention concerns a process for control of a CVT of the bevel-pulley-belt type during an ABS operation or during a lockup braking.
Continuously variable automatic transmissions of the bevel-pulley-belt type, hereinafter referred to as CVT (continuously variable transmission), consist of the following units: a start-up unit, a forward/reverse drive unit, a variator, an intermediate shaft, a differential, and hydraulic and electronic control devices. Such a design has become known from the ATZ Automobiltechnische Zeitschrift 96 (1994) 6, page 380. In a vehicle equipped with a CVT there arise in the practice, during a lockup braking, the following problems: due to the rigid through drive of the CVT the lockup reduces the internal combustion engine to less than an idling speed, the so-called stall. As second effect belt slip can occur as result of the high speed gradients on the primary and secondary pulleys of the variator. In this connection DE-OS 40 42 092 has disclosed changing the ratio of a CVT to a new ratio value during the ABS operation, said new ratio value constituting a function of the ABS operation. DE-OS 40 42 090 has in turn disclosed a process for changing the actual ratio to a targeted ratio within a previuosly calculated time after the end of the ABS operation. The time here corresponds to a deceleration time from the actual speed of the vehicle to stoppage. It is known from EP-A1 0 639 732 to maintain the actual ratio during an emergency stop.
The prior art described regards only the variator as the active system for a braking operation. The prior art thus has the disadvantage that the connection between a vehicle wheel and an internal combustion engine, by a CVT, is not taken into account during a braking operation.
In view of that the invention is based on the problem of providing, for a braking operation, a process which takes into account the whole internal combustion engine/CVT system.
According to the invention the problem is solved by the fact that a wheel-locking drive situation is set when a control device for an antiblocking system indicates an active condition or the gradient of the wheel speeds is above a limit value and a brake is actuated or alternatively a brake pressure exceeds a limit value and the vehicle speed is higher than a limit value. With the setting of the wheel-locking drive situation a converter bridging clutch is opened, simultaneously a forward drive or reverse drive clutch is opened and the pressure level in the secondary pulley is raised, the ratio of the CVT then being changed by a predetermined differential value and the ratio being changed with a predetermined gradient. The solution offers the advantage that during locking braking the drive connection between output and input is severed. Hereby excessive stress of the power train is prevented and by an increase of the secondary pressure, belt slip is effectively prevented.
In a development of this it is proposed that the wheel-locking drive situation be reset when in a first scan it is positively detected that the brake has not been actuated or alternatively that the brake pressure is below a limit value and the wheel speed and the vehicle speed are lower than a limit value. In a second scan a forward or reverse drive position must be positively detected and a throttle valve signal must not exceed a limit value.
In case of a negative result on the second scan, it is to be tested whether the ratio is higher than a limit value. Here the ratio values above the limit value represent the range of comfortable start up and ratio values below the limit value represent the range of the down time adjustment. The second scan is negative, for example, when a drive position is introduced and the accelerator pedal has not been actuated.
In development of this, it is proposed that during an active drive condition of down time adjustment, at a first moment the secondary pressure be raised and the primary pressure reduced to minimum pressure. At a second moment the down time adjustment is terminated, the time interval between the first and second moments representing a function of the temperature of the hydraulic medium and of the ratio at the first moment. Alternatively to this, it is proposed that the down time adjustment be terminated when the secondary pressure shows a significant change. Another variant may consist of the down time adjustment being terminated when a start up ratio is detected by the axial position of the primary pulley.